Diversity receiver



July ll, 1950 H. o. PETERSON DIVERSITY RECEIVER 5 Sheetls-Sheet 1 Filed May 3l, 1946 July 1l, 1950 H. o. PETERSON DIVERSYITY RECEIVER 5 Sheets-Sheet 2 l I INVENTO` larolddfflefom ATTORNEY Filed May 51, 1946 KN EENKMN July 11, 1950 H. o. PETERSON 2,515,055

DIVERSITY RECEIVER Filed May 3l, 1946y 5 Sheets-Sheet 3 INVENTOR lamldjerm ATTO RN EY Patented July 1l, 1195i() 2,515,055 DIvEnsrrY RECEIVER Harold O. Peterson, Riverhead, N. Y., assigner to Radio Corporation ofv America, a corporation of Delaware Application May 31, 1946, Serial No. 673,291

This application concerns receivers for raphy signals or similar signals of low frequency -or relatively slowly varying type including facsimile signals and the like. This application -is in particular concerned with diversity receivers for frequency shift telegraphy signals and the like. l

In frequency shift telegraphy at the transmitter wave energy is shifted inaccordance with signals from a first frequency representing Mark to a second representing Space and vice versa. The first land second frequencies are separated in the frequency spectrum the desired amount depending on the use to which the system is to be put, at least an amount sui'licient to permit the receiver circuits to discriminate between the Mark and Space frequencies, and provide control currents for recording purposes. The energy representing Mark and Space may be of high frequency or of intermediate frequency used to modulate high frequency oscillations or of low frequency, in which case they may be used at the transmitter to modulate higher frequency oscillations. These systems are treated in some respects at least as frequency modulation systems since currents are shifted in frequency by the signals. The mark and spacing frequencies are on continuously but are never on simultaneously during signalling. Currents or potentials representing the mark and spacing frequencies recur at the keying rate and are the low frequency or relatively slowly varying type signals referred to in the first sentence off-this specification. f

The general object of my invention as'disclosed herein is to improve frequency shift telegraphy receivers of the diversity type.

In describing my invention in detail reference will be made to the attached drawings wherein Fig. 1 illustrates by circuit element and circuit connection the essential features of a diversity receiver system arranged in accordance with my 'l'Y Claims. (Cl. Z50-8) telegsignals which at the receiver may be used to 5s operate ink recorders or printing telegraph machines. These two frequencies usually differ by several hundred cycles in frequency. When such a signal is transmitted over a radio circuit the effects of multipath transmission may be such as to cause the wave energyof different frequencies to have considerably different amplitudes most of the time at the receiver. The ratio of the energies at the two frequencies is found to be varying continuously at a given receiving antenna. This variation in relative amplitude is generally known as selective fading. The selective fading at two different antennas spaced on the order of 1000 feet apart has been found to be different, so that the two frequencies may be nearly of equal magnitude at one antenna while they are of radically different magnitude atanother antenna. This phonomenon is known as Space diversity. Then a frequency shift signal received by the spaced antennas #l and #2 might ybe as represented in Figs. 2, 3 and 4, even though the two frequencies are separated by a small frequency band of the order of 1000 cycles or less. For example, the radiant energy pickedv up by antenna #l during the Space interval might be as represented at S and the radiant energy picked up at this antenna during the Mark interval might be of substantially equal intensity as represented in Fig. 2 by M. On the other hand, the radiant energy -pattern at the #2 antenna might be as indicated. in Fig. 4 and then theradiant energy picked up on the #2 antenna might be as illustrated in Fig. 3, with the Space energy of much less magnitude than the marking energy. On the contrary, the marking energy might be much smaller in magnitude than the spacing energy.

It can be seen that when the two frequencies at a given antenna have considerably different amplitudes the result in the receiver is the same as if the signals were amplitude modulated. The amplitude modulation may be removed by the limiter in the receiver but there will be a phase modulation component. Since in these systems a type of frequency modulation is used, i. e.,

signalling is accomplished by frequency shift,-

this phase modulation resulting from limiting adds distortion to the frequency shift in accordance with the signalling wave. This'is proven by the fact that observation showsfthat when thel amount of frequency shift signalling is reduced the amount of distortion due to this induced phase modulation caused by limiting` becomes relatively large.- For example, a limiter input when the antenna #2 is supplying the receiver might be as represented in Fig. 5 where the ordi- In diversity K lation produced during the limiting action as.

illustrated in Fig. 7, wherein the ordinates again represent voltage and the abscissas represent time. The pips riding on the signal voltages. represent distortion which is undesired.

In known diversity receiver systems for frequency shift signals such'as, for example, dirsclosed in Peterson et al. U. S. application Serial #629,298, filed November 17, 1945, now Patent No. 2,494,309, dated January 1o, 1950, and in Schock etal@ U. S. applicationSerial. #632,978,

led December 5, 1945., the energies pickedupby` the two `or more antennas are fed through separate channels to gating tubes biased. to cutoff or otherwise rendered ineffectiveV to repeatv the signal fed thereto. The signalenergies are compared and a potential resulting. from such comparison is used to openthat gate tube supplied with the best signal. Detectionmay take place in each channel and the gate tubes are then excited by the demodulated signals, as in both. of saidA applications or detection may4 take. place. in a stage followingthe gating. tubes. in'which case the gating tubes are excited by the keyed. carrier currents asin Figure. 3 of the rst. mentioned.

application. In these systems the different `versions of the. signals intercepted byl the antenas are comparedA astc carrier magnitudesand that gate tube opened which is4 excited by the signal energy having the. best carrier. The .fre-

quency shiftsignalhavingthe bestcarrier may also be amplitude modulatedin the manner de.-

scribed above and. illustrated in` Figs. 3, landfiso that the used signal maybe the vone having the most undesired phase modulationiin addition tosignal modulation) atthe limiter output.

Thisof.course, results in a` diStOrtedsignal fory p recording use.

A general object of .my invention is to. overcome this defectinknown. diversity receivers. of

frequency shift and similar signals.. This object is attained in .accordancey .withmy inventionby.

comparing the signalspicked up atthe two. ana.

tennas as `to their relative amplitude.mo.dula.. tions, and providing by such comparisonacontrol.

potential which operatesto. open that gatetube supplied by the antenna havingv the least ampli.-V tude modulation thereon.

Referring now to Fig. 1, thetWo antennas orradiant energy pick-up `members. #I and'. 4t2A supply energy to substantially similar receiving. apparatus included in. rectangles I and. lf.v

This receiving apparatus inunits l0. and IB! may comprise. radio frequency. amplifiers, heterodyn.- ing oscillators, IF amplifiers and filters, and if desired, IFheterodyne, and second IF. frequency amplifiers and filters. Inanyevenneach of the;

receivers supplies to the leads l2, and l2,l energy of IF frequency which shifts in .frequency from one frequency representing Mark to a second frequency representing Space VThe frequenciesV involved may be of the order of. 50 .kciFS/z.,y

the. frequency shift as described in saidtwo Vap-k plications referredto. above., The. amplifyingand interina etc... circuits in Hifand.. |10! preferably:

include automatic gain control circuits. These gain control circuits may comprise rectiiiers I4 and I4 connected as shown in alternating current and direct current circuits. The alternating current circuits may include the exciting windings I6 and it and the alternating current bypassing condensers i3 land i8. The direct current circuits comprise, thediode cathode-return and cathode load impedance 2t lwhich supplies the potentials which rise and fall in maghi-- tude depending on the carrier intensity, to one or more amplifying stages in each receiver by way of a resistor 23. The delay or timing of the gain control, actionvv depends upon the size of the condenser 2t. and/or of resistors 2K3, and 23 and is made slow enough to filter out the alternating current but not slow enough to filter out 'variations representing the carrier magnitude changes.

The signals from the antennas #I and #2 are supplied bychannels #l and Y#E1-2 including condensers- 3Q andA 3B( to the control grids 32 and 32 of a pair of gating tubes 34 and 34.. The gating tubes or translating electron control devices 34 `and Sri and the operation thereof may be as disclosed in the said Schock et al. application. It will be assumed that one or the other of the gate tubes or gate devices is open so that the description of these channels may proceed. Say tube 35 is operative to repeat the signal supplied to the grid 32by the condenser 3Q; The signal is amplified and appears across load impedance 38. andA is fed from said load impedance through a coupling condenser d to a current amplitude limiter, amplifiers, a frequency discriminator and a detector system included in unit 42. in either of the channels of said above mentioned applications and will not be described in detail herein. The frequency discriminator and detector output which comprises pulse energy varyingA between two values, one of which may represent Space signatelements and the other of which may represent Mark signal elements is I' supplied to the apparatus,` in rectangle lill which may include a filter, couplingr tubes, a double locking circuit and locking circuit restoring means, all as describedrinsaid Schock et al. application. The output of the locking circuit inunit i4 may be used-tokey a tone source which in turn operates printing or other recording equipment.

Therdistortion described above takes place in the limiter in the apparatus ofcircuit 42 and distorts the output of the` apparatusk at 44 as described hereinbefore@ The outputsof. the receivers #tand #2 are also supplied to a signal sensing or comparing meansI comprising the channel A, the input of which is connected to one receiver, and the channel B, the input of which is connected to the other receiver so that these channels are ex` cited by currents or voltages of intermediate frequency representingy the two versions of the signals intercepted by antennas #E and #2. The two channels A and B comprise wide bandpass filters operating atintermediate frequency and.

.intermediate frequencylampliers `which may be included in units 50. and. 59'. The outputs of these wide band pass Vamplifiers which may be as disclosed in said Schocketal. application are suppliedfto a rectifier arrangement included in.

units 52 and 52'.. In. anyevent the circuits in 50 and in the preceding stages are such` as to" pass al1 important signal :frequencieswithout The apparatus herein may be as disclosed.

ati

frequency selection or descriinination which detected and appear in the rectifier load impedances 56 and 56'. The rectiiiers may be conventional rectiers, and may comprise alternating current circuitsincluding secondaries 60 and 60 and alternating current bypass and ltering condensers `62 and 62". The potentials across the resistances 56 and 56 vary'in accordance with magnitude of the carrier energy of intermediate frequency. These potentials are supf plied thru resistorsl and 10', for automatic gain control purposes, to the amplifiers in 50 and 50. Condensers 'l2 and 12 remove variations of the intermediate frequency and of the modulation frequency which here is the keying frequency so that the gains in the amplifiers 50 and 50 are controlled inversely in accordance with carrier magnitude variations. It should be noted that in the arrangement described the gain control circuits in I0 and 50, 52 assure that the output of the rectifier 'in 52 Varies only in accordance with the amplitude modulations on the received carrier as represented in Figure 3.

The audio variations appearing acrossresistances 56 and -56' are fed by coupling condensers M and 'M' to audio frequency ampliers 16 and 'F6' and then to audio frequency rectiers 80 and 80'. The rectiers 80 and 80 may include diodes with their electrodes coupled in opposed or differential relation by output impedances 86 and 86 shunted by condensers 88 and 88'. The rectiers 80 and 80 may be arranged in audio frequency circuits including transformer secondary windings 8| and 8| and audio frequency bypass condensers 83 and 83 with direct current circuits including the differentially connected impedances 86 and 86'.

It may be safely assumed that the signals never have equal amplitude modulations thereon of the type described hereinbefore so that across diierential resistors 86 and 86 are produced potentials 'of diiferent magnitude so that there is a resultant potential for application to the control grid and cathode of the locking circuit control tube 90. If the signals were amplitude modulated in a like manner the potentials at the two cathodes, i. e., at the points X and X would be of like positive magnitude so that there would be no potential difference applied from these points across the grid to cathode impedance of the tube B0. Then the gate tube 34 or 34 which has been opened will remain open until there is substantial difference in the amount of amplitude modulation on the signals in the two channels. Then the potential at the point X will be either plus or minus relative to the potential at X, depending on whether the signal supplied to channel A has the most or least amplitude modulation thereon.

The manner in which this control potential appearing across the resistors- 86 and 86' is used to open up that gating tube 34 or 34 excited by the signal carrying the least amplitude modulation of the character described above will now be described briefly.

The grid of tube 90 'is coupled to one terminal of the resistors 86 and 86. The cathode of tube 90 is coupled by a resistor 92 to the other terminal of the resistors 06 and 86'. The cathode of tube 90 is also coupled to ground by resistor 94 and a point between the resistors 92 andl=94 is coupled tothe controi grid of a tube lut of the erst-1 stage of a double locking circuit arrangement.v The cathodes of the first locking circuittubes |00 and |02 are coupled to ground by a resistor |04, and the grid of the tube |02 is coupled to ground by a resistor |05. The voltage changes which occur across the resistor 92, here in phase with the voltage change across the resistors 86 `and 86', are applied to the grid of tube |00.

Thetubes |00 and |02 are in a locking or tripping circuit of the type disclosed in Finch Patent #1,844,950. The anodes and control grids thereof are cross-coupled by resistors |06 and l |08 in such a manner that when current starts to flow in one tube it is cut o in the other tube and vice versa. Tubes lill and Htcomprise'a second locking circuit with the anodes and control`grids similarly connected. The control grid of tube ||4 is connected to the anode of tube |00 by a condenser |20, while the control grid of tube H6 is connected to the anode of tube |02 by a condenser |22. The locking circuit cornprising tubes ||i| and ||6 include grid resistors |26 and |28 and cross-coupling resistors |30 and The arrangement is such that whentlie control potential on the grid of tube |00 becomes less negative this tube draws current so that its anode potential drops. This drop in anode potential is supplied to the grid Aof tube |02 by resistor itt and to the grid of tube IM by condenser |20 so that current is then cut off in tube |02 and in tube Hd, and is turned on in tubes itil and llt. The overall operation here is very last since although the potential variations at the anodes oi' tubes |00 and |02 start out slowly' the rinal nlp-over is rapid and the rapidly changing pulse applied through condensers |20 and |22 flips the condition of tubes ||4 and lib' rapidly,

all as described in detail in said Schock et a1.v

application.

The double locking circuits are as in said Schock et al. application, used to vturn on or off one of the tubes 3d or Sil. The grid of tube H4 is connected to ground by a resistor |35 while the grid of tube H6 is connected to ground by va resistor ll. The potentials appearing between the resistors |26 and |36 and between |28 and M0 vary differentially and are applied to the first grids lin the tubes 3d and 34. These tube grids are biased by the potential drops in cathode resistor |52 and are, disregarding the potentials applied thereto by the locking circuit, biased negative to cut on. The differentially varying potentials at the grid ends of resistors |36 and |43 are applied to the first grids of tubes 35 and 34 to bias one thereof to cut off and the other thereof to a less negative value at which that tube is made conductive. The locking circuit as is known rests in the position or condition to which it is tripped until a change in the applied control potential at X, X triggers it to its other position of stability.

In thesaid Schock et al. application referred to above, the locking tube connected with the receiver getting the signal having the'best carrier intensity is opened up. In the present arrangement the signal output of the receiver wherein the received signal has the least amplitude modulation thereon is to be used and it is therefore desired to open up or make conductive the gating Atude modulation, the tube 0|) will draw less cur- 7', rent and-thepotential-onthe grid of tube |00. becomesfmere nega-tive to cut oif current intube and-maketube Il4-conductive and tube Il' nonconductive, then the potential at Y will fall while-the potential at Yri'ses. This fall in pol tential-,at Y` will;bias tube 34. to cutoi while the rise in-potential at Y willmake tube 34' conductive; rLhus tube 34 will supply output from channel ,#2- on-which `there is the least amplitude modulation. Note that the connections between the locking circuit tubes H4 and I I6 and the, control gridsv of the tubes 34 and`v 3'4 are crossed as compared to the corresponding connections. in said Schock et al. application. Reversingthese connections from the. yposition showrikreverses the gating action.

The gating tubes 34 and34 may include Ain theirgrid circuits potential limiting diodes I! and |62 serving as in said prior application to restrict the bias on the first grids of the gating tubes.r

I- claim: t

1. In a frequency shift receiver system, means for intercepting at least two different versions `of a frequency shift telegraphy signal, a translating `electron control device for each Version of the signal, each translating device having t input electrodes excited by electrical energy representing a different one of the said versions'ofV the signal, a common output for all of saiddevices, apparatus. for Abiasing the said devices substantially to cut off so that the devices are unresponsive to the said energy exciting the same, apparatus for comparing the said intercepted. Versions of said signal and for producingY a control voltage signifying which of the intercepted signals has the least amplitude modulation thereon, and apparatus actuated by the control voltage for overcomingthe bias on the translating device excited by the energy representingf the said last mentioned signal. i l

2. A1 diversity receiving system for radio frequency signals the frequency of which is shifted in` accordance with signals comprising in combination,a pair of radiant energy pick-up members, an electron control device for each pick-up member excited by radio frequency energy characterisvtic'of the frequency shifted radiant energy picked up by the corresponding member, said deviceshaving a common output circuit, a current amplitude limiter and frequency discriminator andl detector in cascade coupled to said common output circula apparatus for biasing said devices to cutoi', anY apparatus coupled to each pick up member for generating radio frequency currents characteristic of the radiant energy picked up thereby, a detector coupled to each of said lastnamed apparatuses for detecting amplitude modulation on the respective radio frequency currents, arectifier coupled to each detector for rectifyi'ng the-said amplitude modulations, means connecting the two rectifiers differentially to a common circuit in which the two amplitude modulations are compared, and apparatus coupledto said last-named circuit and to said devices for overcomingv the bias on that device which is excited by characteristicY energy havingY rel() 8 picked up by the corresponding member, said devicesI havinga`- common output circuit, apparatusfo'rbiasing said devices to cutoff, an apparatus coupledto each pick up member for generating. radio frequency currents characteristie-'ofradiantenergy picked up thereby, a detectorfcoupledto each of said last-named apparatuses for detecting. amplitude modulation on the respective radio frequency currents,A a rectier coupledto each detector for rectifying the said amplitude modulations, means connecting the two rectiers differentially to a common circuit in which the two amplitude modulations are compared, and apparatus coupled to said lastnamed circuitv and to said devices for producing a potential for overcoming the bias on that device whichr is vexcited by characteristic energy having the: least; amplitude modulation thereon.

4. Ina frequency shiftl receiver system, means for intercepting at least two different versions of a frequency shift telegraphy signal, a gate device for each version of the signal, connections to each gate device excited by electrical energy representing adiiferent one of said versions of the signal, a biasing circuit for each of said gates to make the devices unresponsive to the said energy exciting. the. same, apparatus for generating. alternatingv current characteristic of each version of said signal, a detector and rectier in cascadey in the order given coupled to each of said last named apparatus', an output circuit, including resistors wherein rectified currents opposa-'coupled to said rectiers for producing a control voltage whichfis of a magnitude depending on which of the intercepted signals has the least amplitude modul-ation, and a control circuit actuated bythe control-voltage and difierentiall'y coupled' to the gate devices for making that gate device excited by the energy representing thesaid'fsignal having the' least modulation eifective to amplify the said energy.

5. In a frequency shift signalling system in combination, a pair of radiant energy pick up members, a tube valve for each pick up member excited by energi/characteristic of the radiant energy picked up by the vcorresponding member, a common output circuit for said valves, an ampli-fier includingautomatic gain control means coupled to each of said pick upy members, a detector and a r'ectier in cascade for each amplier, a wide band'ncoupling between each rectifier and its amplifier, a load circuit for each rectierincluding resistors wherein the outputs of said rectifiers are opposed to derive a potential thei magnitude of'r which depends upon which rectiiierl has the weakest output, and means actuated by said potential for overcoming the bias on said valve connected with that pick up device receiving the-` signal with the least amplitudev modulation thereon.

6. in apparatus for selecting that version of several versions of a signal, represented by currents the frequencies of which are modulated by signals-and the amplitudes of which may vary at random; which has the least amplitude variation, a detector for the current of each version of said signal, a wide' bandpassv couplingv for each detector for impressing the current versions on thev respective detectors; a rectier coupled to each detector for rectifying the detectedv currents, apparatus for comparing the intensities of the rectified currents, and/apparatus coupled with said last named apparatus and actuated by the Sl'fiQ Of: the frequency shifted radiant energy' resultant of the compared currents for selecting and putting to use that version of the signal having the least amplitude variation.

7. A diversity receiving system for frequency shift telegraphy comprising, in combination, at least two spaced radiant energy pickup members, a gate device for each pickup member excited by energy characteristic of the radiant energy picked up by the correspondingr member, said devices having a common output circuit, means for biasing said devices to cutoff, an amplifier with automatic gain control connections coupled to each pickup member, an amplitude modulation detector coupled to each amplier, an amplitude modulation rectier coupled to each detector, said rectifiers being differentially connected to a common impedance, and a locking circuit coupled to said common impedance and to said devices to control the biases thereon.

HAROLD O.` PETERSON.

10 REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,004,107 Goldsmith June 11, 1935 2,089,409 Ohl Aug. 10, 1937 2,249,425 Hansell July 15, 1941 2,253,832 Whitaker Aug. 26, 1941 2,253,867 Peterson Aug. 26, 1941 2,282,526 Moore May 12, 1942 2,290,992 Peterson July 28, 1942 2,333,335 Peterson Nov. 2, 1943 2,384,456 Davey Sept. 11, 1945 2,388,052 Hansell Oct. 30, 1945 2,414,111 Lyons Jan. 14, 1947 2,420,868 Crosby May 20, 1947 2,441,661 Crosby May 18, 1948 

